When liquids are stored in tanks, any entrained solid components can drop out, this is a particular problem with crude oil since crude oil inevitably includes entrained solid components, such as sand or silt, that are removed from the subterranean oil reducing formation as the oil flows or is pumped to the earth's surface. When crude oil is stored in a tank, it normally sits in a relatively still, dormant state for a long period of time. Gravity pulls the solid particles down and over time creates a layer of sludge on the tank bottom. If not stirred so that the sludge is re-suspended into the tank fluid, the sludge is not removed as the crude oil is removed from the tank and will continually build up on the tank bottom and become a serious problem.
The accumulation of sludge on the bottom of a crude oil storage tank can cause a number of operational problems. One serious problem is that the capacity of the storage tank is reduced. Another example is as various layers of sludge deposits form, they may entrap pools of water which later form hydrates or water slugs in the outflow from the tank. When a tank employs a floating roof, a common expedient in tank storage technology, the floating roof typically has legs which limits the downward travel of the floating roof as fluid is removed from the tank. Sludge can cause uneven landing of the legs. Another problem is that when sludge builds up as a consequence of storage of crude oil, it may interfere with the use of the tank for storage of a different type of fluid product.
For all of these reasons, sludge accumulated during normal operations of a storage tank must be removed. If it is not removed by re-suspending the sludge or bottom sediments in the tank fluids so that it is removed as a normal consequence of the outflow of the tank fluid, it is sometimes necessary to physically remove it. This is a costly problem since it is a hazardous occupation for workmen to enter into the interior of a tank to assist in the physical removal of sludge. Further, the disposal of large amounts of accumulated sludge becomes an environmental problem and, therefore, an expense to the operator.
For this reason, it has been a practice of the operators of storage tanks and particularly crude oil storage tanks, to employ systems that recirculate tank fluid through a jet system, the jet serving to discharge tank fluid at high velocity over the tank bottom to cause the sediments to be re-suspended in the fluid. An example of a sludge removal machine of this type is illustrated in U.S. Pat. No. 4,407,678 entitled "Sludge Removal Machine" that issued Oct. 4, 1983. The machine described in this patent provides oppositely directed nozzles and is an example of previously existing efforts to which the present invention is directed as an improvement. For further background information to various tank cleaning devices, reference may be had to the following previously issued U.S. Pats.:
______________________________________ Title U.S. Pat. No. ______________________________________ Apparatus and Method of Cleaning Tanks 1,978,015 Containing Fluid Apparatus For Cleaning Tanks and The like 2,116,935 Apparatus For Cleaning Tanks and The like 2,647,639 Method and Apparatus For Cleaning The 2,991,203 Interior Of A Tank Tank Washing System 3,121,027 Liquid Jet Producing Device 3,408,006 Automatically Cycling Swimming Pool Cleaning 3,449,772 System Part Circle Water Motor Driven Sprinkler 3,523,647 Sedimentation Tank With Rotary Sediment 3,542,207 Raking Structure Pressure Jet Tank Cleaner 3,544,012 Method and Apparatus For Creating A Suspension 3,586,294 of Fine Particles In A Liquid Pop-Up Head For Water Jet-Pool Cleaning 3,675,252 System Continuous Sedimentation Tank With Center- 3,704,789 Pier Supported Sediment Raking Apparatus Apparatus For Cleaning Tanks And The Like 3,878,857 Method and Apparatus For Cleaning Vessels 3,895,756 Tank Cleaning Apparatus 3,953,226 Swimming Pool Cleaner 4,347,979 Sludge Removal Machine 4,407,678 Method For Cleaning Settled Sludge 4,685,974 Liquid Circulator Useful For Dispersing Sediment 4,945,933 Contained In A Storage Tank Process For Recovering Crude Oil Or Refinery 5,078,799 Products From Sludgy, Thickened or Sedimented Products Method For Dispersing Sediment Contained In A 5,091,016 Storage Tank Remote Controlled Sludge Removal System 5,138,741 Remote Controlled Sludge Removal Apparatus 5,269,041 ______________________________________